Conventionally, there has been known a control device for a continuously variable transmission with an auxiliary transmission which performs a cooperative control in which a variator and a stepped transmission mechanism are simultaneously shifted (cf. for example, a patent document 1). Moreover, there has been known a control device for a continuously variable transmission of a vehicle in which the cooperative control is not performed (stopped), and the 2→1 shift speed is set higher than that of the cooperative control for rapidly starting to increase the driving force, in a depression downshift control with a shift of the auxiliary transmission mechanism of the continuously variable transmission with the auxiliary transmission (cf. for example, patent document 2).
The “cooperative control” is a shift in (by) which the shift speed of the variator is increased so that the shift speed of the variator corresponds to the shift speed of the auxiliary transmission, and in which the transmission gear ratio of the variator is varied in a direction opposite to the variation direction of the transmission gear ratio of the auxiliary transmission, when the gear stage of the auxiliary transmission mechanism is varied. By performing the shift by this cooperative control, it is possible to suppress the sudden variation of the transmission gear ratio of the entire transmission (hereinafter, referred to as “through transmission gear ratio”), to decrease the shift shock before and after the shift by the auxiliary transmission, and thereby to suppress the unnatural feeling to the driver.
However, in the conventional device, at the depression downshift control with the shift of the auxiliary transmission, the downshift (2nd speed→1st speed) of the auxiliary transmission is performed at a speed higher than that of the cooperative control. In a case where the transmission gear ratio of the variator is higher than the lowest transmission gear ratio, the variator is once downshifted from the high side to the lowest side. The variator is upshifted from the lowest side to the high side in accordance with the shift of the desired target through transmission gear ratio.
In this way, when the variator is downshifted from the high side to the lowest side, the transmission gear ratio of the variator is shifted and overshot to the side lower than the control lowest transmission gear ratio which is set for the control. Accordingly, when the variator is started to be upshifted from the lowest side in accordance with the shift of the desired target through transmission gear ratio, “Low separation shock” is generated, so that the unnatural feeling is provided to the driver.
It is, therefore, an object of the present invention to provide a control device for a continuously variable transmission with an auxiliary transmission which is devised to solve the above-described problems, to suppress the Low separation shock generated in the variator at the depression shift control with the shift of the auxiliary transmission mechanism, and to improve the drivability.